Abstract
Accurate PET quantitation requires a matched attenuation map. Obtaining matched CT attenuation maps in the thorax is difficult due to the respiratory cycle which causes both motion and density changes. Unlike with motion, little attention has been given to the effects of density changes in the lung on PET quantitation. This work aims to explore the extent of the errors caused by pulmonary density attenuation map mismatch on dynamic and static parameter estimates. Dynamic XCAT phantoms were utilised using clinically relevant 18F-FDG and 18F-FMISO time activity curves for all organs within the thorax to estimate the expected parameter errors. The simulations were then validated with PET data from 5 patients suffering from idiopathic pulmonary fibrosis who underwent PET/Cine-CT. The PET data were reconstructed with three gates obtained from the Cine-CT and the average Cine-CT. The lung TACs clearly displayed differences between true and measured curves with error depending on global activity distribution at the time of measurement. The density errors from using a mismatched attenuation map were found to have a considerable impact on PET quantitative accuracy. Maximum errors due to density mismatch were found to be as high as 25% in the XCAT simulation. Differences in patient derived kinetic parameter estimates and static concentration between the extreme gates were found to be as high as 31% and 14%, respectively. Overall our results show that respiratory associated density errors in the attenuation map affect quantitation throughout the lung, not just regions near boundaries. The extent of this error is dependent on the activity distribution in the thorax and hence on the tracer and time of acquisition. Consequently there may be a significant impact on estimated kinetic parameters throughout the lung.
Highlights
Quantitative PET images can only be obtained with accurate attenuation correction (AC) maps
Overall our results show that respiratory associated density errors in the attenuation map affect quantitation throughout the lung, not just regions near boundaries
Mismatches due to motion have been widely explored in the literature (Nehmeh and Erdi 2008, Sun and Mok 2012) and density variations due to the respiratory phase at the time of acquisition have long been acknowledged by the CT community (Shaker et al 2004, Gorbunova et al 2012), its effect on PET quantitation has been largely ignored
Summary
Quantitative PET images can only be obtained with accurate attenuation correction (AC) maps. These AC-maps are commonly derived from short CT acquisitions and are appropriate in regions where internal anatomical motion is unlikely (Beyer et al 2003, Osman et al 2003). In regions such as the thorax, these CT techniques only produce a ‘snap shot’ of the respiratory cycle and determining a PET matched AC-map is difficult (Pepin et al 2014). Mismatches due to motion have been widely explored in the literature (Nehmeh and Erdi 2008, Sun and Mok 2012) and density variations due to the respiratory phase at the time of acquisition have long been acknowledged by the CT community (Shaker et al 2004, Gorbunova et al 2012), its effect on PET quantitation has been largely ignored
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